Standalone Shell source

Where can I find the source for the standalone shell that is being
run when the I boot up the kernel?
-Tom
P.S. progress on dz11 driver - transmit generates interrupts that
produce characters on the serial line - however, the other direction
seems not to generate interrupts. (i.e. receiving characters doesn't
seem to work)
-----------------------------------------------------------------------
Given enough eyeballs all bugs seem shallow.